From Ancient Ayurvedic Herb to Modern Supplement Phenomenon
Ashwagandha (Withania somnifera) has been used in Ayurvedic medicine for thousands of years as an adaptogen — a substance that helps the body adapt to stress and restore physiological balance. In the last decade, it has become one of the world’s best-selling dietary supplements, propelled by a growing body of clinical research that validates several of its traditionally ascribed properties. Among the most compelling findings is ashwagandha’s impact on the hormonal system — particularly cortisol, testosterone, and thyroid hormones.
Ashwagandha and Cortisol Reduction
The most robust evidence for ashwagandha’s hormonal effects involves cortisol reduction. Multiple randomized controlled trials have demonstrated that ashwagandha supplementation significantly reduces cortisol levels in stressed adults. A widely cited double-blind RCT published in Medicine found that 300 mg twice daily of a standardized ashwagandha extract (KSM-66) reduced cortisol levels by approximately 28% compared to placebo over 60 days, with corresponding significant reductions in perceived stress scores. Similar findings have been replicated across multiple trials using different ashwagandha extracts and populations.
The mechanism appears to involve modulation of the HPA axis — ashwagandha’s withanolide compounds appear to influence hypothalamic and pituitary signaling, reducing the cortisol output in response to stressors rather than simply blocking cortisol’s peripheral effects. This is clinically important: it addresses the upstream hormonal regulation rather than just downstream cortisol levels.
Ashwagandha and Testosterone
Multiple clinical trials have found significant testosterone-increasing effects of ashwagandha in men. A well-designed 8-week randomized trial of KSM-66 ashwagandha in resistance-trained men found 15% higher testosterone levels compared to placebo, alongside significantly greater muscle strength and recovery. Another trial in stressed but otherwise healthy men found testosterone increases of approximately 15–17% with ashwagandha over 8 weeks. The proposed mechanism involves both direct effects on testicular testosterone synthesis (ashwagandha appears to stimulate Leydig cell function) and indirect effects through cortisol reduction — since cortisol chronically suppresses testosterone, reducing cortisol raises testosterone as a consequence.
Ashwagandha and Thyroid Function
A notable but less widely discussed effect of ashwagandha is its influence on thyroid hormone levels — specifically, increases in T3 and T4 in subclinically hypothyroid individuals. A double-blind RCT found that 600 mg/day of ashwagandha root extract over 8 weeks significantly increased serum T3 and T4 concentrations and reduced TSH in subclinically hypothyroid patients. The mechanism is not fully understood but may involve enhanced peripheral conversion of T4 to T3 or stimulation of thyroid hormone synthesis. For individuals with subclinical hypothyroidism, ashwagandha may be a meaningful adjunctive intervention — though it should not replace medical evaluation and treatment when clinically indicated.
Choosing the Right Ashwagandha Supplement
Not all ashwagandha supplements are equivalent. The clinical evidence is concentrated on standardized root extracts — primarily KSM-66 (standardized to 5% withanolides, using only root) and Sensoril (standardized to 10% withanolides, using root and leaf). These proprietary extracts have been used in most of the positive clinical trials. Generic ashwagandha products of unstandardized potency may not provide equivalent effects. Effective dosages in clinical trials typically range from 300–600 mg twice daily. Ashwagandha is generally well-tolerated with a low side effect profile — occasional GI upset and, in rare cases, thyroid hormone elevation in people with existing hyperthyroidism.